M. Gasgnier

Chemical syntheses by means of microwave digestion as a focused open-vessel system: structural properties of oxides and hydroxides as powders

Abstract By means of a focused microwave (monomode system) apparatus, it is possible to carry out some chemical syntheses in very short times. Various kinds of oxides and hydroxides have been irradiated by this method. Different transition phases and different reduction and oxidation reactions have been observed in the course of the heating treatments. The most important results are provided by black powder, which absorbs the microwave energy and quickly becomes incandescent red at low temperature. However, the absorptance is a function of the dielectric constant and dielectric loss values, which vary from one oxide to another. Compared with some previous results obtained using multimode closed-vessel domestic ovens, the focused system allows us to homogenize the heating inside the powder and to reach higher temperatures in the cases of a few absorbent materials (particularly white powders).

Possible superconductivity and microstructural investigation in Y1−xCaxSrCu2.6Fe0.4O6+z (x = 0, 0.1)

Abstract Preparation, X-ray diffraction, electron diffraction and microscopy, and energy dispersive X-ray analyses of Y1−xCaxSrCu2.6Fe0.4O6+z (x = 0, 0.1) are described. Zero resistivity was observed at 10 K for x = 0, which increased to 25 K for x = 0.1. These were confirmed by AC susceptibility measurements. Possible observation of superconductivity in this system is discussed.

In situ studies of microstructural changes in YBa2Cu3O7−x during O and He irradiation

Abstract We have studied the microstructural changes occurring in YBa2Cu3O7-x (the so-called ‘123’ phase) upon room temperature irradiation by He+ and O +, in situ, on samples that had been thinned by ion milling before irradiation, using a transmission electron microscope on line with the ion implanter. In previous work on samples crushed from the bulk or on films ion-milled after irradiation, the samples amorphized above a threshold of about 0.3 displacement per atom. In the present case, no amorphization is observed, instead, precipitation of Cu2O occurs, indicating complete demixing of the ceramic. This process seems to nucleate at the ion-milled surfaces. For a displacement-per-atom ratio of about 0·10, a change from the orthorhombic to the tetragonal structure of the ‘123’ compound occurs. It is suggested that the result is quite general. The effects of the microstructural changes on the electrical properties are briefly discussed.

Irradiation-Induced Orthorhombic-To-Tetragonal Phase Transition in RBa2Cu3O7-x (R = Eu, Gd)

Thin crystals of RBa2Cu3O7-x (R = Eu, Gd) were irradiated with 50 keV He+ ions at 15 K and 300 K, and their structure was studied in situ via transmission electron microscopy. An orthorhombic-to-tetragonal phase transition was observed (at both irradiation temperatures) at fluences around 4 1015 He+/cm2. Amorphization occurs in the LT-irradiated samples at fluence ≥ 1016 He+/cm2. Radiation-induced damage (dislocation loops) was also identified, for the first time, in the orthorhombic phase at irradiation fluences in the (1 ÷ 3) 1015 He+/cm2 range.

In situ structural evolution of ion-irradiated R─Ba─Cu─O (R = Eu, Gd) superconductors

Abstract We report an in situ transmission electron microscopy study of damage evolution Eu—Ba—Cu—O and Gd—Ba—Cu—O ceramics under 50keV He ion irradiation. Our results on the irradiation-induced orthorhombic to tetragonal phase transformation (OTT) complement previous work. We report observations on the existence and evolution of defect clusters at 15 and 300 K, as well as the first evidence for the interaction of defects with pre-existing dislocations. We have also studied the dependence of the irradiation-induced amorphization threshold on the cation species and on the irradiation temperature. Taken together, these results lead us to propose the existence of at least two types of irradiation defects: an anionic defect (associated with oxygen) and a cationic defect. The former (which is mobile at 15 K) leads to defect clustering and interaction with dislocations; it is also related to the OTT. The latter is less mobile and is presumably instrumental in the amorphization process. We also point out that th...

New developments in the field of energy transfer by means of monomode microwaves for various oxides and hydroxides

Abstract By means of a “focused” microwave oven (monomode system) it is possible to carry out chemical syntheses within very short times. The use of a new magnetron with higher performance experimental conditions has allowed us to observe phenomena which have not previously been recorded. For instance, it is possible within very short treatment times to form copper metal from Cu 2 O, TiO 2 rutile from TiO 2 anatase, PbO from Pb 3 O 4 and the V 6 O 13 and V 2 O 5 higher oxides from V 2 O 3 . Finally, it is possible to turn white powders such as SnO 2 , TiO 2 and ν-Al 2 O 3 and green powders such as Cr 2 O 3 incandescent red. Several hypotheses will be put forward to try to explain this phenomenon. The temperature measurement is also discussed as a function of the dielectric constant of the materials.

Macro- and microstructural analysis ofRBa2Cu3O7−x (R =Y, Gd, Er, and Tm) superconductor thin crystals between 300 and 15 K

Abstract Some macro- and microstructural crystallographic properties of the new superconductors RBa 2 Cu 3 O 7−x (R =Y, Gd, Er, Tm) have been studied. Chemical composition of some uncrushed monocrystalline ErBa 2 Cu 3 O 7− x have been determined by scanning electron microscopy fitted to an energy dispersive X-ray spectrometer. Conventional transmission electron microscopy with a liquid He cooled stage showed that only ∼25% of small crushed crystals have defected zones as dislocations or planar defects. Electron diffraction patterns point out the presence of the orthorhombic and sometimes of the tetragonal structures of YBa 2 Cu 3 O 7− x . Other patterns show the presence of various known compounds (i.e., BaCuO 2 ) and unknown compounds often in coexistence with the well-established orthorhombic RBa 2 Cu 3 O 7−x .

Structural change induced in Kr irradiated RBaCuO ceramics (R Gd, Ho)

Abstract Crushed RBaCuO ceramics (R = Gd, Ho) were irradiated with 480 keV Kr ions at 40 and 300 K. In situ electron microscopy was performed during irradiation from 5×10 11 to 2.5×10 13 Kr/cm 2 . The large deposited energy has two effects: (i) it leads to amorphization at low (≈0.02) displacement per atom ratios, presumably due to cation “backbone” disordering; (ii) it produces visible extended defects which interact each other and with the twin boundaries. This leads to twin boundary deformation and dislocation loops formation lying in the Cu O basal plane.

CuxY compounds as thin films: crystallographic and compositional analyses of yttrium rich phases

Abstract The interdiffusion of Y films deposited onto Cu substrate by flash evaporation and sputtering was studied (concentration profiles, X-ray and electron diffraction patterns) in the temperature range 373–553 K. In samples deposited by flash evaporation the first phase to be detected is CuY at 393 K. At higher temperatures an intermediate phase, close to the Cu3Y2 compound, is formed before the Cu2Y stoichiometric phase is produced at 513 K. Crystallographic data confirm the formation of this intermediate Cu3Y2 phase (orthorhombic unit-cell) as resulting from the reaction CuY+Cu2Y→Cu3Y2. The results are identical for sputtered Y films deposited under a cryogenic vacuum (2−5×10−7 Pa). When Y is deposited under a standard vacuum (2−5×10−6 Pa), the influence of a diffusion barrier at the Cu–Y interface is noticed. This influence is characterized in the concentration profiles by an Y concentration hump at the Cu–Y interface and a depletion of Y at the front of the profile. Three main phases are observed. They correspond to the Cu5Y2, Cu2Y and Cu3Y2 compounds.

Structural, electrical and magnetic properties of Y-Sr-Cu-O

Preparation, resistivity (ρ), field-cooled (FC) and zero field-cooled (ZFC) susceptibility (χ) of YSrCuO are reported. Zeroρ is observed at 72K. FCχ shows a superconducting transition at 68 K and also an antiferromagnetic transition at 15 K which is due to Y2Cu2O5 as deduced from electron diffraction and composition analyses. It is tentatively proposed that the Y-rich phase could be responsible for the observed superconductivity.